Misfiring and backfiring when engine is warm

[Gufshoz]

I believe I have it down to a fuel issue. I just switched from a Tillitson to a rebuilt Zenith. On the Tilly I ran with the GAV turned from about 1/4 to 1/3 turn from closed. Reading the forum input, most of you also run your Zeniths about the same. I haven’t noticed any difference when I open the GAV a full turn. Should I give it 2 turns and then check the plugs for fouling?
Because it only occurs when warmed up and I have a new manifold I’m thinking that might be the issue. I can’t think of anything else that would change from the expansion of being heated. I tried spraying starting fluid around the manifold, but I didn’t get any increase in rpm’s. Do most of you have the manifold milled true by a machine shop? If so, do you need to have the exhaust manifold done at the same time?

[chap52]

I don't have time to read the entire post right now, but have you checked that fuel cap vent? When it is acting up unscrew the gas cap and if you hear a "hiss" your cap is not venting. This causes fuel starvation , backfire and sputtering. Went through that adventure awhile back. Chap

[Gufshoz]

Yup. I checked the gas cap. I can see daylight through the vent and easily blow through it.

[Fairview]

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

[30 Closed Cab PU]

Originally Posted by Fairview

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

If the issue returns, wireless modern distributor plates create all kinds of odd issues including yours. If you have them convert back to original style.


I did not see wireless plates mentioned previously. I had issues similar to yours, spent 3 weeks trying things, finally someone asked me if I was running wireless plates and I converted back to original - problem solved.

[Marshall V. Daut]

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[Fairview]

Originally Posted by Fairview

I’ve been following this thread with keen interest lately as I had the same symptoms with my 29 Sport Coupe.

Summary is that it would start and idle, but after warming up and heading down the street I could feel it losing power, surging and popping, backfiring and not revving up. If I turned the GAV about a full turn, I could limp back home, but that was about all I could do. I checked everything with the carburetor, fuel supply, then ignition including coil polarity, no joy.

Today I replaced the nice old Ford script coil with a generic 6 volt coil for testing purposes and took a test drive. It runs great. So in my case, it was the coil that was the culprit. What threw me off the scent and had me thinking fuel issues was the improvement when opening the GAV.

I guess I’ll order another Ford script coil, but I’m going to run with the $19 one from O’Reilly for a while.

I just wanted to follow up on my own recent post. Several miles of driving today, several stops and starts all confirm the problem with my Model A was most certainly the coil.

It sure runs sweet and reliably now.

[Nick in Fayetteville]

I never had a fuel problem that caused the car to backfire, Fuel problem will cause rough running and stalling. My bet... the coil

[30 Closed Cab PU]

My truck will backfire/cough at the carb if running too lean on teh GAV, especially when shifting and slowing down. The only time I have had backfiring exhaust is when I was running Nurex wireless distributor plates, they were causing poor connection in the points circuit, mostly at low speeds or when shifting - cured by converting back to original design plates.

[Purdy Swoft]

If A spark plug wire-strip is laying over and actually contacting the distributor cap body it can melt a spot under the plug wire and short out at higher RPM and cause rough running . It took a long time to find the problem . One night I was worrying with it and saw a spark near and under the number four plug wire . When I removed the plug wire I could see a slightly melted spot on the new distributor cap body . The car would crank and idle perfectly but would begin to run rough and lose power at about 40 MPH or above . The insolation of the cap body is too thin on some of the new distributor bodies and can short out if the plug wire is laying over and contacting the distributor cap body . Replacing the distributor cap body and using longer model B plug wires solved the problem . Some spark plugs are shorter than others and cause the plug wires to contact the distributor cap . As the RPMs increase the spark gets hotter and can short through the thinner insulated distributor caps . If this is really the problem there will usually be a small spot under where the plug wire has slightly began too melt the cap . The melt spot was not visible until the plug wire was removed .

[katy]

Does anyone have the lengths of the A and B spark plug "wires".

[Don S]

Katy, 28-29 3 3/8 ". 30-31 3 1/4" model b 3 13/16 " I personally like the model b "wires " because they are thicker than the model A ones. Your purchase could be different. Good luck Cheers

[Tudortomnz]

I have had this problem recently [ and previously] & always electrical. Last time , lack of power , backfire when warm. I pulled the points, had minor pitting so replaced with a good [ original] set. Problem fixed. Just because the gap is OK & timing good, does not mean points are good . A Model A will run OK with carb & float not right & fuel really was dirty in the old days, not like today & with all the add on filters people now put on. Cheers.

[katy]

Originally Posted by Don S

Katy, 28-29 3 3/8 ". 30-31 3 1/4" model b 3 13/16 " I personally like the model b "wires " because they are thicker than the model A ones. Your purchase could be different. Good luck Cheers

Thanks.